Combating Occupational Heat Stress: Getting Past the Basics

Adequate hydration, nourishment, acclimatization, and equipment are factors at work here.

By Kris Bancroft

Jul 01, 2003

THIS article is intended to help the employer and safety professional to further enhance an existing heat stress prevention strategy. In reading this article, keep in mind that each workplace should have its own custom-designed heat stress prevention program; there is no such thing as a "one size fits all" strategy because every workplace has its unique situations and needs. The ultimate aim of a heat stress prevention strategy is to keep the workers' core body temperature from rising above its normal level and approaching 105 degrees F, which is where heat stress injuries begin to occur.

The Basics
Almost everyone is aware that exposure to high levels of heat is, to some varying degree, detrimental to human health. Most of us know there are various levels of heat stress. Almost everyone understands that heat stress is lessened by taking rest periods to allow the body to cool down, and that workers must be properly nourished and hydrated.

Some authorities suggest, via an extrapolated work-to-rest regimen, that no work should be performed at temperatures greater than about 92 degrees F. People in the "real" world know that this is not practical. While environmental conditions may be hot, there is nonetheless work that must be done.

While it is true the work-to-rest cycle must provide more time for rest during hot (and other adverse) conditions, we know that healthy human beings, when properly equipped, nourished, and hydrated, can safely perform work so long as the heat index is below 130 degrees F. Remember: Only workers who are adequately hydrated, nourished, acclimated to the heat, properly equipped, and in robust health can work for appreciably long periods when the heat index nears 130 degrees F!

Often, the work accomplished in hot environments requires extraordinary physical effort. In some cases, there are psychological demands that accompany this work, and these mental demands add to the possibility that a heat-related injury might occur. So, what can the employer do to decrease the likelihood that a heat-related incident might occur in the workplace?

Crucial Data
The most important thing an employer can do is to know the full range of temperature and humidity of the workplace. If the conditions are prone to significant variations, some device or method must be incorporated--and continually monitored--to ensure the worker is not exposed to excessive temperatures.

Temperature and humidity are combined to calculate the heat index--an analog for determining how heat and humidity affects us. Calculating the heat index requires numerous mathematical steps, so it is more practical to refer to a heat index chart. (see Table A).

Formerly, the standard method for measuring the heat index was the "wet bulb/dry bulb/globe temperature" (WBGT) method. Today, there are numerous electronic devices on the market that are exceedingly more accurate and user-friendly.

Table A

HEAT INDEX CHART

Actual Temperature--Fahrenheit

75

80

85

90

95

100

105

110

115

120

125

Relative
Humidity

0%

69

73

78

83

87

91

95

99

103

107

111

10%

70

75

80

85

90

95

100

105

111

116

128

20%

72

77

82

87

93

99

105

112

120

130

145

30%

73

78

84

90

96

104

113

123

135

148

40%

74

79

86

93

101

110

123

137

151

50%

75

81

88

96

107

120

135

150

60%

76

82

90

100

114

132

149

70%

77

85

93

106

124

144

80%

78

86

97

113

136

90%

79

88

102

122

138

100%

80

91

108

136

The second-most-important factor the employer must realize is that there are limitations to human endurance, and each individual has a unique endurance level. Furthermore, an individual's endurance level can and does change.

Sunlight increases the heat index. Anyone who has ever touched the hood of a car that has been sitting in the sun on a hot day knows this to be true. Just as a car or building will absorb heat, so do people. How much? It depends.

To derive precise values for solar load, one would need to employ some fairly complex equipment and perform some serious mathematical computations. It is sufficient to the purposes of this article to consider the following: The solar load that may be imposed on a worker is determined by the relative position and distance of the sun to a given latitude and longitude on the Earth, and to the degree which sunlight passes through the atmosphere. On a bright, sunny, mid-summer day in the Florida Keys, an average increase in the heat index would be perhaps 5 degrees F. Under similar circumstances in Portland, Maine, the solar load may increase the heat index by only 2 degrees F.

Hint: Most of the time, for outdoor workplaces, the employer can access weather conditions reported by the local media.

Air Currents, Composition
Years ago, someone came up with the concept of a convection oven. The principle that makes a convection oven cook more efficiently is that it stirs hot air around the food. The convection process begins to cook items when the temperature rises to about 94 degrees F, hence, when the ambient temperature reaches this point, fans alone provide no assistance.

Hint: Work in confined spaces requires the additional assessment of atmospheric constituents--oxygen levels, hydrocarbons, particulate, and the like. Variances from normal atmospheres are known to affect the way the body regulates heat.

UV radiation causes skin cancer. Blue-eyed people are more prone to skin cancers caused by UV exposure. Outdoor workers should wear long-sleeved garments and broad-brimmed hats or use a UV blocker. SPF 15 is adequate for most skin areas, but SPF 25 should be applied to the nose and ears. If UV blocking lotion is applied to the forehead, perspiration will carry it into the eyes; it is thus more practical to tie a bandanna about the forehead.

The stories about frying eggs on sidewalks or keeping warm by leaning against a building are examples of how radiant energy works. In essence, objects absorb heat. This is why one should continue to monitor workers for heat stress even after the sun has set. Thus, if temperature- and humidity-measuring devices are used, they should be located either close to the work or near objects that radiate heat.

Planning the Work Day
As indicated earlier, the mission of the employer is to get the job done, and this often necessitates that the worker performs tasks in hot environments. Planning the work day so that more-strenuous tasks are accomplished during the coolest parts of the work cycle will help prevent the occurrence of heat-related injuries. It may require some fancy logistical moves on the part of the employer, but it is infinitely better to juggle work schedules a bit than to place a worker at avoidable risks.

You must consider the insulative capabilities of the garments worn by the workers. Wearing heavy or tightly woven garments (e.g., synthetics) raises a worker's the core body temperature. Unless working conditions dictate otherwise, loosely woven cotton garments are best; they promote the wicking of perspiration, which then may evaporate and cool the skin. Loosely woven cotton also allows air to flow through the weave. It follows that workers who must wear protective clothing must also be allowed more frequent and longer cool-down periods.

Hint: As humidity rises, the evaporation rate decreases, and thus perspiration loses its ability to cool the skin.

The employer should make an analysis of limitations of the workforce. This begins by compiling a medical profile of each worker. The kind of data one seeks includes a history of sentinel health events, including, but not limited to: diabetes; heart, liver, respiratory, and kidney conditions; obesity; drug use; alcoholism; epilepsy; and especially previous episodes of higher-order heat stress injuries. Once an individual has suffered a sentinel heat event, he or she is forever more susceptible to suffering a heat-related injury again.

Acclimatization and Diet
It generally takes about five days for a worker to become acclimatized to performing work in hot environments. The employer should allow new workers to gradually build their resistance to heat. One way to do this is to allow the new worker to work only two hours under hot conditions on the first day, adding two hours to his hot work cycle each subsequent day.

Hint: When workers have been away from the hot work environment (e.g., for illness or vacation), they must be given an opportunity to reacclimatize themselves.

During periods when the heat index is high, workers should be cautioned against eating heavy meals (e.g., fried foods, large quantities of meat, and sugary drinks.) The body cools itself by circulating blood to the surface of the skin where, under optimal conditions, the evaporating perspiration cools the blood. The cooled blood is then circulated to the body core, thus reducing the core body temperature. When the circulatory system becomes taxed with digesting a heavy meal, its ability to cool the skin is hindered.

There are many electrolyte drinks available today. While these drinks may be good for most people, workers who have diabetes or suffer from hypertension may be harmed by these products. Drinking plenty of cool water is the best way to stay hydrated. The employer is required to furnish to its employees a reasonably convenient supply of potable water and a sanitary means of dispensing it to them.

Hint: The question of precisely how much water someone should drink while working in a hot environment is always asked. A good way to measure this is to make sure the worker has a surplus of water intake. A worker should urinate at least once per hour when working in extreme heat; this is a way to make sure the kidneys are functioning normally.

What to do if a Worker Experiences Heat Stress
Miliaria (the medical name for "heat rash") is the mildest form of heat stress. It usually occurs when the skin is constantly wet and chafed by wet clothing. It becomes dangerous when the red bumps on the skin turn yellow and become infected. The effects of heat rash can be minimized by applying talcum powder to the affected area and having the worker change into clean, dry clothing.

Simple heat cramps occur in the extremities of the body and indicate that the serum level of calcium has dropped below normal. Increasing the intake of calcium and promoting its assimilation into the body by increasing potassium intake will reduce the occurrence of heat cramps. A banana milkshake after work is an excellent treatment for heat cramps.

Syncope (heat fainting) is third in the ascending level of heat stress concerns. Workers suffering heat fainting should first be examined to ensure they have not been injured during their fall, and then (if no injuries have occurred) assisted as necessary into a cooler area so they may recover. Fainting is not technically a loss of consciousness; the pupils of a person who has fainted will be sensitive to light. Additionally, persons who have fainted tend to regain their senses almost immediately and will try to move around.

Heat tetany, heat edema, and heat stroke are sentinel injuries. These conditions constitute a medical emergency of the highest order--seek medical attention immediately! In the interim, move the person to a cooler area. If he is conscious, give him cool water to drink, about two ounces at a time. Make him lie still. Apply ice packs (if available) to the head, neck, underarms, and inner thighs. If no ice is available, bathe the victim in cool water and fan him to increase the evaporation rate of the water.

The symptoms of these conditions may include seizures, loss of consciousness, stomach cramps or spasms, anaphylactic shock, clamminess of the skin, and cessation of sweating. It is crucial that the core body temperature of persons suffering heat tetany, heat edema, or heat stroke be reduced as quickly and safely as possible.

Summary
During the summer months, and throughout the year in foundries, steel mills, and other workplaces where the temperatures reach high levels, workers get the job done; doing so fulfills the mission of the employer. Workers do their work and return home injury-free because they have made a conscious decision to work safely and because the employer has enabled them with the proper tools and equipment to work injury free.

Heat stress injuries, like any other form of workplace injury, can be prevented. It is incumbent upon the employer to provide the means that allow the worker to avoid injury, and it is incumbent upon the employee to follow the plan set forth by the employer. The information in this article, it is hoped, can assist those who work in hot environments to beat the heat.

This article originally appeared in the July 2003 issue of Occupational Health & Safety.